Refactor how AOTAutograd backends are defined (#89736)
There was a lot of strangeness in how AOTAutograd backends were previously defined. This refactor replaces the strangeness with something simple and straightforward. The improvements:
- There is no longer a footgun aot_autograd "backend" which doesn't actually work. No more mistyping `torch._dynamo.optimize("aot_autograd")` when you meant "aot_eager"
- Deleted aot_print because it's annoying and anyway there's no uses of it
- Instead of having BOTH the backend Subgraph and AotAutogradStrategy, there is now only an aot_autograd function which takes the kwargs to configure AOTAutograd, and then gives you a compiler function that does AOTAutograd given those kwargs. Easy.
- The primary downside is that we are now eagerly populating all of the kwargs, and that can get us into import cycle shenanigans. Some cycles I resolved directly (e.g., we now no longer manually disable the forward function before passing it to aot_autograd; aot_autograd it does it for us), but for getting inductor decompositions I had to make it take a lambda so I could lazily populate the decomps later.
New code is 130 lines shorter!
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89736
Approved by: https://github.com/anjali411, https://github.com/albanD
